The term SMA is applied to a group of metallic materials which, when subjected to appropriate thermal loading, are able to return to a previously defined shape or size. Generally an SMA material may be plastically deformed at some relatively low temperature and will return to a pre-deformation shape upon exposure to some higher temperature by means of a micro-structural transformation from a flexible martensitic phase at the low temperature to an austenitic phase at a higher temperature. The temperature at which the transformation takes place is known as the activation temperature. In one example, a TiNi alloy has an activation temperature of approximately 70° C. An SMA is “trained” into a particular shape by heating it well beyond its activation temperature to its annealing temperature where it is held for a period of time. In one example, a TiNi alloy is constrained in a desired shape and then heated to 510° C. and held at that temperature for approximately fifteen minutes.
In the field of medical devices SMA materials, for example TiNi alloys, such as Nitinol, or Cu alloys, may form a basis for actuators designed to impart controlled deformation to elongated interventional devices. Examples of these devices include delivery catheters, guide wires, electrophysiology catheters, ablation catheters, and electrical leads, all of which require a degree of steering to access target sites within a body; that steering is facilitated by an SMA actuator. An SMA actuator within an interventional device typically includes a strip of SMA material extending along a portion of a length of the device and one or more resistive heating elements through which electrical current is directed. Each heating element is attached to a surface of the SMA strip, in proximity to portions of the SMA strip that have been trained to bend upon application of thermal loading. A layer of electrically insulating material is disposed over a portion of the SMA strip on which a conductive material is deposited or applied in a trace pattern forming the heating element. Electrical current is directed through the conductive trace from wires attached to interconnect pads that terminate each end of the trace. In this way, the SMA material is heat activated while insulated from the electrical current. It is important that, during many cycles of activation, the insulative layer does not crack or delaminate from the surface of the SMA strip.